The invention disclosed herein pertains generally to hydrostatic transmissions, and more particularly to a control device for a hydrostatic transmission.
Hydrostatic transmissions are well known and are used, for example, in land vehicles. Typically, a hydrostatic transmission includes a prime mover, such as an internal combustion engine, a variable displacement pump mechanically powered by the prime mover, and a fluid motor powered by the variable displacement pump. In addition a fixed displacement pump, called a charge pump, powered by the prime mover replenishes any hydraulic fluid used in the hydrostatic transmission which may be lost as a result of leakage.
Control systems for hydrostatic transmissions are also known. U.S. Pat. No. 3,152,445 to Weisenbach, for example, discloses a hydrostatic transmission having a speed control system which ensures that a fluid motor unit does not exceed a predetermined speed. The position of the cam plate of the variable displacement pump of the hydrostatic transmission is regulated by a manually actuable position responsive servo control. The Weisenbach position responsive servo control includes a servo valve which fluidically controls a piston which is mechanically linked to the cam plate. The position of the cam plate is fed back to the servo valve by a mechanical linkage to maintain the cam plate at a position selected by the manual input to the servo valve.
To ensure that the fluid motor speed does not exceed a predetermined maximum value, and to ensure that this speed is maintained substantially constant at the predetermined maximum value, an orifice control device is provided at the outflow of the fluid motor to sense its speed by sensing a pressure drop across a bore. This pressure drop is communicated to a speed control valve which controls the fluid communication between a charging pump and the servo valve to limit the fluid motor speed by positioning the cam plate and renders the manual control ineffective. Thus, the Weisenbach control device senses the speed of the fluid motor in order to conform the displacement of the variable displacement pump to a predetermined maximum speed of the fluid motor.
Another control device for variable displacement pumps is that disclosed in U.S. Pat. No. 4,158,529 to Nonnemacher et al. The Nonnemacher et al device prevents overshoots and undershoots of the adjustment of the output of a variable output pump for changes in the speed of the prime mover. The adjusting element of the variable-output pumps is controlled by a slide valve whose position is controlled by the output of a pump which represents the speed of the prime mover. A mechanical feedback signal from the adjustable element of the variable output pumps to the slide valve prevents over and undershooting.
Yet another control device for a variable displacement pump is that disclosed in U.S. Pat. No. 3,871,177 to Ridlen. In part, the Ridlen device regulates the displacement of the variable displacement pump in accordance with the speed of the prime mover by sensing the pressure of the discharge from a gear pump driven by the prime mover. This pressure is communicated to a piston of a pilot spool in order to urge the pilot spool to move to a position which, in part, is representative of the speed of the prime mover, to thereby vary the displacement of the variable displacement pump. The motion of the pilot spool uncovers a port resulting in pressurized fluid being communicated from the gear pump to an annular piston encircling the pilot spool to urge the annular piston to undergo a motion corresponding to that of the pilot spool. That is, the motion of the pilot spool results in a fluid signal from the gear pump being communicated to the annular piston, to urge the annular piston to undergo a corresponding motion.
Other control devices for hydrostatic transmissions are those disclosed in the following patents: U.S. Pat. No. 2,630,681 to Ferris; U.S. Pat. No. 3,996,743 to Habiger et al; U.S. Pat. No. 4,047,590 to Hoashi et al; U.S. Pat. No. 4,094,145 to Habiger; U.S. Pat. No. 4,182,125 to Spivey, Jr.; and U.S. Pat. No. 4,203,712 to Uehara.
While the prior art does disclose various types of control devices for hydrostatic transmissions, the prior art does not disclose a control device for a hydrostatic transmission like that of the present invention.